This is an extension of an earlier discussion, but more geared towards actual encounter design, I had a spark of genius (so to speak). When a good idea was brought up that got lost in all the posts and I feel it deserves it's own discussion.

Tier Discussion, Class Balance Discussion or anything similar will NOT be tolerated in this thread.

The main idea behind this thread is to give creative ways to make encounters difficult for our spellcasting players, without making them feel as if they are completely useless. A lot of these ideas I've seen used in pre-generated adventures, especially Worlds Largest Dungeon.

#1 - Use of Silenced areas in a dungeon. This does not indicate traps. I'm talking about areas of the dungeon where noise does not do anything. Silence (as the spell) only gives a save if it is cast on a person. The effect (if triggered by a trap, or if it's just an ongoing effect suppresses all noise with no save.

#2 - Force concentration checks as a result of spooky dungeon features. Perhaps there is a constant cold draft that keeps the characters from being comfortable and staying warm. Perhaps it imposes a penalty on all concentration checks and forces concentration checks whenever a spell is cast. Maybe there are odd sounds in the dungeon?

#3 Use intelligent guards, perhaps there is a BBEG and he obtained a lock of hair. Now he can scry on the group, or send enemies at them.

#4 Wild Magic zones/Planar travels. - Use of planar magic can get very fun. Perhaps spells of a certain type dont function at all (common in planar settings). Or perhaps whenever a spell is cast it has a chance to produce a random effect that is uncontrollable by the player. These can get very fun, maybe the mage tried to haste his allies but instead he polymorphed them?

#5 Use grapplers - sure the arcanists might be able to dimension swap/dimension door away but at least you can try and pin them in a hold. Particularly useful in ambushes/outdoor encounters especially at lower levels. (Crocodiles!)

Anyone have any other ideas for making encounters that specifically challenge the casters or party more as a whole than simply throwing monsters at the group?

Don't forget the simple act of differing the strategies of enemies. Having them stand far enough apart that they aren't fried in one fireball forces to use of more spells, having them ambush the party can put casting classes closer than they might like, and having reinforcements arrive the next round can keep every foe from being slain by one mighty spell.

And naturally, counter-spells. On one hand, you can't have a caster in every fight, but when you do it probably makes sense that they decided most of their best spells are the same ones the party is using.

I keep seeing aboleths being ganked in melee combat as it charges out to bite a party member. WTF?

Aboleths have a crapload of Illusion spells as at will abilities. Just *finding* an Aboleth in his lair should involve multiple encounters dealing with illusionary everything : walls, floors, monsters, people, spell effects, etc. That before you can even find the darned thing. Assuming it hasn't taken the time to flee.

#10 - Enemies with multiple mobility options: spider climb, levitate, anklets of teleportation (transposition? whatever those things are), etc. Can at least have one or two enemies with readied actions to escape area-of-effect spells if the players do manage to get them all lined up conveniently.

Most of these can be summed up as "anything you can do, I can do better."

#11 - Include spellcasters among enemy ranks who specifically target spellcasters among the players.
#12 - In the case of arcanists, hit them in their relatively weak fort save. Options include Blindness/Deafness, Stinking Cloud, and my personal favorite, Baleful Polymorph.
#13 - At higher levels, include an enemy using Greater Dispel Magic. For extra effectiveness, pair them with a grappler with some means of teleportation or another means of high mobility, so that they have a chance to catch the mage without Freedom of Movement up. (If players have been scried, this can include rings.)

#1 - Use of Silenced areas in a dungeon. (...) Silence (as the spell) only gives a save if it is cast on a person.

#1.1. Silence is a 20' radius emanation. If you cast it on a point in space, the enemy spellcasters can just move away; you loose a spell, they loose a round, period. If you cast it on the spellcaster, they will probably save. Then cast it on your mage slayer, this rogue who can tumble past the tank guys and continually threaten the wizard. Bonus point if the rogue is also invisible, because he can continually sneak attack. Or use it on your grappler.

#14. Combine traps and creatures. Have your kobold sorcerer and/or archers position themselves behind a Camouflaged Pit-Trap. The fighter PCs want to charge them? Oops, he falls 30' lower, and is roasted for this fight...

Had a bbeg lieutenant mage with a wand of fireballs once who decided to see how many charges he could spend on the party without them catching him. Pop up on a balcony, fireball the party once or twice, exit while the mooks close in and distract them.

As an extension of this, not every mook mob is going to contain a caster, but those that do will have either very smart or very cunning casters--they'll set traps (can be as simple as, "All right you all, grab a crossbow and line up behind this door. Take aim and fill the next thing to come through with bolts. Those of you who can't hit a man-sized target at 10 meters, grab a spear or a spiked chain and stand behind the archers.").

Importantly, smart or cunning casters will cut their losses when it looks like they're losing. You might only have 5 casters between 100 mooks, but if those 5 keep running away and making reappearances (and possibly even start working together) it'll seem as if you have way more casters--possibly even 15 or 20 between those 100 mooks.

At low levels especially, casters are only good for a few turns anyway; they get off (say) a web, a fireball, and a grease and then they're done, it's time for them to cover their retreat with a dozen kobolds. Next encounter, that same caster (with another dozen kobolds) might get off a second web, a hold person, and a magic missile or two, and then run away again. Your players will start cursing his name at that point (especially if he has a wand of fireballs). And you still only had one caster between 25 mooks.